- Add request deduplication to getOrFetchUser using RequestDeduplicator
- Consolidates multiple simultaneous calls for same user into single API request
- Prevents duplicate 401 errors and wasted network traffic
- Matches pattern already used by fetchUsers and fetchUserStats
- Remove backwards compatibility aliases (getOrFetchUserByName, getOrFetchUserByUsername)
- Update all call sites to use canonical getOrFetchUser method

Fixes issue where PagedMemoList, useMemoViewState, MainLayout, and UserProfile
were making duplicate user fetch requests when loading user data.

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Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>